The present invention generally relates to ceramic matrix composite (CMC) materials and articles produced therefrom. More particularly, this invention is directed to method of forming a CMC article with a protective outer barrier layer that prevents damage to near-surface reinforcement material within the article.
CMC materials generally comprise a ceramic fiber reinforcement material embedded in a ceramic matrix material. The reinforcement material serves as the load-bearing constituent of the CMC in the event of a matrix crack, while the ceramic matrix protects the reinforcement material, maintains the orientation of its fibers, and serves to dissipate loads to the reinforcement material. Of particular interest to high-temperature applications are silicon-based composites, such as silicon carbide (SiC) as the matrix and/or reinforcement material. SiC fibers have been used as a reinforcement material for a variety of ceramic matrix materials, including SiC, TiC, Si3N4, and Al2O3. Continuous fiber reinforced ceramic composite (CFCC) materials are a type of CMC that offers light weight, high strength, and high stiffness for a variety of high temperature load-bearing applications. A CFCC material is generally characterized by continuous fibers (filaments) that may be arranged to form a unidirectional array of fibers, or bundled in tows that are arranged to form a unidirectional array of tows, or bundled in tows that are woven to form a two-dimensional fabric or woven or braided to form a three-dimensional fabric. For three-dimensional fabrics, sets of unidirectional tows may, for example, be interwoven transverse to each other. The individual tows may be coated with a release agent, such as boron nitride (BN) or carbon, forming a weak interface coating that allows for limited and controlled slip between the tows and the ceramic matrix material. As cracks develop in the CMC, one or more fibers bridging the crack act to redistribute the load to adjacent fibers and regions of the matrix material, thus inhibiting or at least slowing further propagation of the crack.
One technique for fabricating CMC's involves multiple layers of “prepreg,” often in the form of a tape-like structure, comprising the reinforcement material of the desired CMC impregnated with a precursor of the CMC matrix material. The prepreg must undergo processing (including firing) to convert the precursor to the desired ceramic. Prepregs for CFCC materials frequently comprise a two-dimensional fiber array comprising a single layer of unidirectionally-aligned tows impregnated with a matrix precursor to create a generally two-dimensional laminate. Multiple plies of the resulting prepregs are stacked and debulked to form a laminate preform, a process referred to as “lay-up.” The prepregs are typically arranged so that tows of the prepreg layers are oriented transverse (e.g., perpendicular) to each other, providing greater strength in the laminar plane of the preform (corresponding to the principal (load-bearing) directions of the final CMC component).
Following lay-up, the laminate preform will typically undergo debulking and curing while subjected to applied pressure and an elevated temperature, such as in an autoclave. In the case of melt-infiltrated (MI) CMC articles, the debulked and cured preform undergoes additional processing. First the preform is heated in vacuum or in an inert atmosphere in order to decompose the organic binders, at least one of which pyrolyzes during this heat treatment to form a carbon char, and produces a porous preform for melt infiltration. Further heating, either as part of the same heat cycle as the binder burn-out step or in an independent subsequent heating step, the preform is melt infiltrated, such as with molten silicon supplied externally. The molten silicon infiltrates into the porosity, reacts with the carbon constituent of the matrix to form silicon carbide, and fills the porosity to yield the desired CMC component.
Examples of SiC/Si—SiC (fiber/matrix) CFCC materials and processes are disclosed in commonly-assigned U.S. Pat. Nos. 5,015,540, 5,330,854, 5,336,350, 5,628,938, 6,024,898, 6,258,737, 6,403,158, and 6,503,441, and commonly-assigned U.S. Patent Application Publication No. 2004/0067316. An example of a CFCC material is depicted in FIG. 1, which represents a surface region of a CFCC component 10 as comprising multiple laminae 12, each derived from an individual prepreg that comprised unidirectionally-aligned tows 14 impregnated with a ceramic matrix precursor. As a result, each lamina 12 contains unidirectionally-aligned fibers 15 encased in a ceramic matrix 16 formed by conversion of the ceramic matrix precursor during firing and melt infiltration.
In order to maximize the mechanical properties of CMC's, particularly the prepreg MI-type of CMC, it is important to have the reinforcement material well dispersed within the composite matrix. As evident from FIG. 1, such a dispersion inherently places a fraction of the reinforcement material near the surface of the composite where it is susceptible to damage due to handling, machining, surface preparation for subsequent processing steps such as deposition of an environmental barrier coating (EBC), or oxidation attack at high temperature. A cross-section of a prepreg MI-type CMC containing dispersed reinforcement fibers is shown in FIG. 3. A significant number of reinforcement fibers can be seen relatively close to the surface (within about 0.005 inch (about 125 mm), and therefore susceptible to damage.
In view of the above, it would be beneficial to protect near-surface reinforcement materials in CMC's. However, doing so must not compromise the mechanical, thermal, or structural properties of the composite, and must be chemically and thermally (identical thermal expansion) compatible with the bulk of the CMC material.